1. Field of the Invention
The invention relates to methods and apparatus for controlling or measuring the thickness of material layers which are accessible only from one side or not at all, wherein a continuous, frequency-modulated ultrasonic signal that periodically scans a frequency band is transmitted and wherein the signal resulting from interference is evaluated with respect to the amplitude.
2. Prior Art
Measurement of layer thicknesses is necessary for controlling thicknesses of coatings, such as protective layers or deposits, corrosion of tube wall thicknesses, and for depth determinations and many other applications. Optical, electric and magnetic methods are known for these applications, but these are limited to specific materials and frequently cannot be used for inaccessible layers below the surface.
Contactless and nondestructive testing by means of ultrasonics permits even such regions of a workpiece to be examined which are inaccessible from outside. Ultrasonic thickness measurements based on the evaluation of propagation delay using the pulse-echo technique can be made by means of commercially available apparatus. If the layer thicknesses are, however, so low that the echos from the upper and lower layer surfaces superpose to give one single signal whose structure cannot be resolved in the time domain, the conventional propagation time method fails. In such cases frequency analyzing methods have already been applied wherein the layer thickness was determined from the so-called Cepstrum of the resulting signal. As the pulse duration cannot be arbitrarily reduced, the propagation time method is only applicable to a limited extent, i.e., to relatively thick layers, and the Cepstrum method is calculation-intensive and thus involves a high expenditure of time and cost.
In addition, ultrasonic thickness measuring instruments are known which are equipped with a sound emitter and a HF generator whose frequency is continuously varied with in specific ranges. These involve measurement of the frequencies at which resonance occurs and determination of the layer thickness from these frequencies. It is possible to make a sound audible in the head phone in case of resonance. The relevant frequency or layer thickness can then be directly read off from a numerical display. In these methods the sound cause by resonance gives no indication of the layer thickness. Furthermore, if the layer thickness has to be determined on larger areas or distances, the observer can only get an impression of the result if he continuously observes the oscilloscope screen, other indicating instruments or a record. These methods are too time-consuming for fast routine inspections.